The present invention relates to a rotary shutter, and more particularly to a rotary shutter having stopping means which cause the rotary shutter to stop at a desired position where a shutter opening of the rotary shutter fully exposes an exposure aperture of a camera.
Generally, rotary shutters incorporated in electronic still cameras have a circular array of shutter openings on their shutter blades. This type of rotary shutter is driven by means of a servo-driving system to rotate the shutter blade at a constant speed in synchronism with driving-signals for scanning an image pickup element or image sensor usually placed behind an exposure aperture of the camera, in order to open and close intermittently the exposure aperture. After having reached a constant speed of rotation, the rotary shutter opens the exposure aperture simultaneously with every periodically generated vertical synchronizing signal which is contained in the image sensor driving signals for scanning. Consequently, it suffices to start the image pickup operation at the same time a first vertical synchronizing signal is generated after the actuation of a shutter button. In other words, there is no requirement to cause the synchonized opening of the exposure aperture with the vertical synchronizing signal every image pickup operation. This is of the particular advantage in achieving a favorable operation. In addition, continuous rotation of the rotary shutter makes it possible to achieve drastic reduction of time intervals at which consecutive still images are taken.
Twin blade rotary shutters are well known in the art of motion picture cameras as a said rotary shutter. This type of rotary shutter is of particular advantage in applications where it is necessary to vary shutter time or speed. For varying shutter speed, the twin blade rotary shutter is adapted such that two shutter blades are coaxially superposed for rotary displacement relative to each other in order to change the effective size shutter openings defined between the opposite edges of the openings of the respective shutter blades so as to select the proper shutter speed in accordance with brightness of ambient light and the subject to be photographed. As a result of changing the effective size of the shutter openings, a proper amount of light is allowed to pass through the exposure aperture, reaching the photoelectric conversion surface of the image sensor for an appropriate time.
The above-described type of twin blade rotary shutter used in electronic still cameras is also of particular advantage in applications where it is necessary to maintain the rotary shutter stopped so as continuously and fully to open the exposure aperture. Typically, this is often required when subjects with a low brightness are photographed or in the case of little ambient light or electronic still cameras having diaphragms incorporated therein. In the electronic still cameras of the type having the above-described twin blade rotary shutter, it is possible not only to pick up consecutive still images one for each vertical synchronizing signal contained in the image sensor driving signals for scanning but also to use the camera as a video camera for motion pictures or as a monitor camera.
The rotary shutter described above has disadvantages which make it difficult to implement it in certain environments. In particular, it may be mechanically difficult to stop the rotary shutter at the proper position. A rotary shutter having shutter openings whose size is large enough fully to open the exposure aperture is required to stop exactly at a position where any one of the shutter openings is in alignment with the exposure aperture. For this requirement, the rotary shutter is engaged, after stopping, by a mechanical positioning means in order to relocate the stopped rotary shutter in a correct position. Because of this relocation, it takes a relatively long time to perform the shutter stop operation which includes varying the size of the shutter openings to open fully the exposure aperture and adjusting the rotary shutter so as to bring any one of the shutter openings into alignment with the exposure aperture. In other words, electronic still cameras having a rotary shutter of this type are confronted with an increased time consumption problem when they are changed from an image pickup mode in which images are picked up by rotation of the rotary shutter (hereinafter referred to as the shutter rotating mode) to another mode in which images are picked up when the rotary shutter stops (hereinafter referred to as the shutter stopping mode).